How much time passes between each of the books?

Answer 1

Bilbo was born on September 22, 2890, and Frodo was born on September 22, 2968. The story opens just a few days before the party that was on September 22, 3001. On the day of the party Frodo turns 33 and Bilbo turns 111. It is mentioned that when Frodo leaves the Shire, he was the same age as Bilbo when he left on his adventure which is 50. Frodo left the Shire on September 23, 3018, so that is 17 years and a few days.

From the time Frodo leaves the Shire with the One Ring to the time the Hobbits return, just over a year passes. The Battle of Bywater was on November 3, 3019.

From that time to the time Frodo, Bilbo, Gandalf and the last protectors of Middle-earth leave the Grey Havens on September 29, 3021 is almost 2 years.

So from the very beginning to the very end, just over 20 years elapse.